1. Field of the Invention
The present invention relates to a method of micro/nano imprinting patterning on a non-planar substrate, and in particular, relates to a method which applies soft mold, pre-shaping sealing film, and soft holder arrangements, with the exploring of UV light, or heat treatment, to the micro/nano structure imprinting process of the curved substrates. The method of the present invention prevents the curved surface from crumbling, which may result from high gas pressure, and can obtain uniform imprinting pressure distribution throughout the whole curved substrate. Moreover, replicating micro/nano structures onto a double-sided non-planar surfaces, both convex and concave, can also be achieved.
2. Description of Related Art
Micro- and nano-fabrication have increasingly become more important due to the impending requirements of modern science and technology. The technologies, such as hot embossing, UV-embossing and injection molding etc., enable low-cost mass production of microstructures. As such, it is an appropriate technology for the production of both components and modules for micro optical devices. The general fabrication processes involve the fabrication of an original sub-micron structure relief, the formation of a suitable mold from this relief and the mass production of replicas in polymer (or other materials to be used) from this mold.
Patterning sub-micron structures onto non-planar substrates recently has drawn much interest, especially for the fabrication of dish solar concentrators, optical, and astronomical and bio-sensing components. There are several ways, such as optical lithography, e-beam lithography, ion-beam lithography, X-ray LIGA, laser direct writing and diamond turning, for patterning sub-micron structures down to the curved surfaces. However, most these methods are not applicable because of very short depth of focus, not to mention they are time consuming, complex and expensive.
Nanoimprint Lithography (NIL) is not limited to the resolution of optical diffraction, and has the advantages of low cost, high efficiency, and high resolution, thus is extensively applied to micro/nano patterning. The method of nanoimprint lithography utilizes a mother mold having a pre-determined pattern, and transfers the pre-determined pattern of the mother mold to the resist with a provided pressure. There are two kinds of operating procedures of nanoimprint lithography: thermo nanoimprint and UV cured nanoimprint. Thermo nanoimprint method applies heat treatment to the thermoplastic resist such as PMMA (poly-methylmethacrylate) or PS (poly-styrene) to a moldable state, then cooling to solidify the resist after being in close contact with the mother mold. Finally, the mother mold and the resist are separated to provide a sub-micron patterned structure. UV light exposure is used in the UV cured nanoimprint method to cure the photo-resist at a lower temperature, comparing with the high temperature of the thermo nanoimprint method. The nanoimprint method can reproduce very high-resolution sub-micron structure features. For example, it is relatively easy to replicate into polymer foil sub-micron structure with line width below 100 nm. Besides, although the fabrication of a mold with complex sub-micron structure can be relatively expensive, the replication process results in a commercially competitive process and low cost per replica.
Traditional Nanoimprint Lithography methods were easily performed such as the method described in U.S. Pat. No. 7,137,803, wherein a fluid pressure imprint lithography method is presented. The method uses direct fluid pressure to press a mold into a substrate supported film, in which the stack of the mold, the moldable layer, and the substrate was placed in a flexible, fluid-impermeable membrane (e.g. a plastic bag). In 2007, Heidari et al. proposed a method of nanoimprint, in which a base plate was used to transfer a substrate to a template for nanoimprint process (U.S. Pat. No. 7,195,734). Also, in 2006, Chen et al. (US patent No. 2006/0246169A1) disclosed a method using a uniform pressing unit having a flexible membrane and a fluid filled in a space formed by the flexible membrane, therefore the uniform pressing unit can directly apply pressure to the mold. However, in between these methods, only flat substrates can be applied into fabrication of nano structure objects, and no mention of curved nanoimprints was disclosed in these prior arts.
Alternatively, Cheng et al. disclosed a soft mold and gasbag pressure (GBP) mechanism for patterning submicron patterns onto a large concave substrate, but the method still had some limitations in processing conditions (F. S. Cheng, S. Y. Yang, S. C. Nian and Lon A. Wang, “Soft mold and gasbag pressure mechanism for patterning submicron patterns onto a large curved substrate,” Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, Volume 24, Issue 4, pp. 1724-1727(2006)). In this GBP method, a gasbag is expanded in the chamber and gradually contacts the imprinting patterns. Therefore, non-uniform pressure will occur because the gasbag is not in contact with the imprinting patterns at one time. Besides, the gasbag used in such mechanism has low heat resistance, so the method is always limited to UV light exposure. Since the gasbag used is usually opaque, a transparent substrate (e.g. glass, quartz) for light exposure from the bottom will be needed. Moreover, it is difficult to provide a huge gasbag if a large area imprinting process is performed. Hence, the gasbag pressure mechanism is difficult to apply to semiconductor fabrication because the silicon substrates used usually possess a large area.
Therefore, it is a present need to develop a novel method of micro/nano fabrication with low cost, large area production, and high efficiency, and particularly, to develop a method of providing a double-sided patterned curved structure.